Flywheel energy storage systems (“FESS”) have been known in the art for a number of years, and have proven to be extremely useful for generating, storing and recovering kinetic energy. A conventional flywheel system 10 is depicted in FIGS. 1, 1A and 1B, and includes a flywheel assembly that is disposed within a flywheel housing 14.
A typical flywheel assembly includes a rim 16, a hub 18 that secures the rim to a shaft 20, as well as one or more bearing assemblies 22 (e.g., one or more top bearings 22A and bottom bearings 22B) that rotatably support the shaft. The rim 16 can also be designed to provide structural support to the inner hub 18, in the event that the latter fractures.
In these figures, the radial and axial directions (with respect to a long axis of the shaft 20) are denoted, respectively, by lines C-C and D-D.
In operation, a motor 24 drives the shaft 20, which, in turn, drives a flywheel 26 at a high velocity, thus transforming a significant amount of electrical energy into kinetic energy (which is proportionally related to the mass, and the square of the velocity of the rotor) that is stored within the flywheel assembly. Once the flywheel 26 has attained a desired rotational velocity, the motor 24 is thereafter powered as needed to maintain the rotational velocity of the flywheel within a predetermined range.
Flywheel energy storage systems 10 have found use in numerous environments including, but not limited to short term energy storage and load leveling or smoothing applications. Their use as stand-alone supplemental, auxiliary, or emergency energy sources (i.e., uninterruptible power supplies), however, is particularly preferred.
This is because the design of the flywheel assembly ensures that when the motor 24 shuts down (e.g., due to a power outage), the stored kinetic energy in the assembly will enable a flywheel rotor 28 to continue to rotate for an extended period of time. Rotation of the rotor 28 following cessation of power to the flywheel 26 allows the flywheel to produce/generate power by induction (i.e., electrical power) through the motor 24, which is set in so-called “generator mode” upon sensing input power loss, and which, therefore, can provide short-term, auxiliary or emergency power until power is restored or supplied by other means. The motor 24, when set in the “generator mode,” also can be used to slow down the rotor 28, e.g., for rotor maintenance or in response to a sensed abnormal condition.
Among the most significant concerns with respect to flywheel systems 10 is their behavior in the event of a failure (i.e., “crash”), which generally occurs either when the rim itself fails (thus leaving no barrier for other flywheel parts) due to the flywheel entering an over-speed condition, due to exceeding of low cycle fatigue capability, and/or due to the accumulation of imbalanced stresses, or when one or more of the flywheel parts fails and causes damage to the rim 16 and/or penetrates or otherwise damages the housing 14. In the case of such failure, the motor 24 also typically is not available to slow down the rotor.
Because of the high speeds of flywheel motors 24 and rotors 28, and the amount of kinetic energy that flywheel assemblies 26 store, any failure or crash of the flywheel system has the potential to cause the flywheel assembly to violently break apart, with various sized pieces being scattered at high velocities into the flywheel housing 14, which, if it breaks, could allow the scattered pieces to cause further damage to other persons and/or property.
Fortunately, flywheel systems 10 that are used as uninterruptible power supplies can be sited and operated below grade (i.e., underground). This all but ensures that if any conceivable structural failure of the system 10 occurs, such a failure would likely be entirely contained below ground. Unfortunately, despite these advantages, below-ground siting of flywheel systems 10 is not without problems. For example, such placement hampers/complicates the ability to recognize that a flywheel component has failed, and severely limits the ability to inspect the system 10 for any visual evidence indicating, and/or predicting the onset of failure of one or more of the components of the system.
Generally, equipment that undergoes operational stresses similar to those of flywheel systems 10 is primarily tested for evidence or indicators of failure via visual-based inspections. For example, safety personnel inspect (either with the naked eye, or through the use of visual enhancement devices/techniques) such equipment at predetermined intervals to determine whether cracks have formed on any of the equipment parts, and/or whether some parts appear to be susceptible to crack formation in the future.
In many cases, the safety critical components of the equipment have (or are perceived to have) a longer life than the usuable life of the equipment as a whole, and, consequently, are inspected often only during the manufacturing process. This approach is used for flywheel systems 10 that are sited belowground, where visual inspection during the lifetime of the system is highly impractical to the point of being impossible. Simply put, it would take far too much time and effort to excavate a flywheel system 10, to thoroughly examine it, and then to properly place it back in its belowground location. Expenditure of such time and effort, even in the name of safety, is too costly to operators and too disruptive to operation of the flywheel system 10.
Realizing this, most in the art have attempted to ensure/validate the safety of flywheel systems 10 instead through modeling/experimentation. In accordance with such efforts, flywheel systems 10 are caused to catastrophically fail in simulated usage environments, and are retrieved and painstakingly forensically examined to determine any relevant information regarding the extent of damage caused as a result of their crash (e.g., which parts failed, in what order, at what time, and for what reasons). This information is then compared and contrasted with previous results to estimate acceptably safe system operation ranges.
Unfortunately, this type of safety modeling/experimentation is replete with problems and disadvantages, mostly owing to the fact that, at best, it amounts to educated guesswork. For even after several catastrophic failures are induced, each of which takes considerable time, money and manpower to set up, execute and forensically analyze, the initial suppositions about the specifics of these induced flywheel failures/crashes, may or may not be entirely correct. Only by performing numerous controlled experiments (i.e., akin to Design of Experiments (DOE), Taguchi et al.) and/or advanced structural analysis and failure techniques could one confirm these failure suppositions. Doing so, however, would amount to the expenditure of even more time, money and effort.
Because of this, operators seemingly have been left with little choice but to operate flywheel systems 10 in sub-optimal manners in hopes of decreasing the likelihood of the occurrence of catastrophic failures/crashes. For example, in the name of safety, flywheels systems 10 are usually operated at speeds well below their ideal (let alone maximum) operating speeds and, as such, produce less kinetic energy than is ideal, and much less than is possible to produce.
In essence, conventional flywheel systems 10 are currently being operated in a manner whereby they definitely store less kinetic energy than is ideal/desirable, in an effort to potentially decrease the likelihood of a catastrophic failure, which may or may not have occurred had the flywheel system been operated at its ideal capacity. Therefore, a need exists for a flywheel system that can operate at levels that more closely approach optimal operation levels, while not unacceptably compromising the overall safety and operability of the flywheel system.